It's debatable on the newer model MES with higher heat wattage elements if cooking time for small loads increase cooking time. In the older MES with the lesser wattage elements it is clear that increased load size lengthens the cook/smoke time. I can tell you as an example when I cook 3 to 6 racks of baby back ribs in my MES 40 which is older 800w model, the 221 works great, however I have had to add 1 to 3 hours for more racks of ribs, I have cooked as many as 14 racks of baby backs, even when I say time was added, it should be noted that I usually pull the foiled ribs and don't do the one hour finishing in the smoker, I hit the ribs with a light coat of BBQ sauce seal in their foil packs and finish them on a grill, sometimes as long as 3 or 4 hours later depending on the party schedule.
One thing that does add to the cooking time is opening the hatch. I try to limit that to once when I foil, another time to finish by adding sauce. Thats it two times during the whole smoke. However as I have said above I usually finish on the grill.